Add error when template is passed to generate site

src/main.rs

@@ -392,6 +392,9 @@ fn run() -> Result<(), failure::Error> {
         let site = matches.is_present("site");
 
         let (target_type, template) = if site {
+            if matches.value_of("template").is_some() {
+                failure::bail!("You cannot specify a template when generating a Workers Site. If you want to generate site boilerplate, run wrangler generate --site")
+            }
             // Workers Sites projects are always Webpack for now
             let target_type = Some(TargetType::Webpack);
             let template = "https://github.com/cloudflare/worker-sites-template";